I am only 20 and my memory is so horrible. i will tell myself to do something and forget 30 seconds later. what is wrong with me? what should i do?

Not dementia: More than likely at your age you are not developing dementia, which is a mental health condition involving degeneration of the brain matter controlling memory and decision making. Most likely you are having issues with anxiety or depression. Either of these conditions can cause perceived memory loss due to problems paying attention. I recommend that you start by seeing your primary care doctor

Stress?: Memory loss of this kind is not unusual if you are a very busy person or under some form of stress. Certain medications may also cause this.Evaluation may be needed for this and de-stressing techniques learned.Memory improvement exercises can help so also certain foods rich in antioxidants such as berries red grapes broccoli, foods rich in vitamin B and omega 3 fatty acids such as salmon.